In recent years, compact self-ballasted fluorescent lamps are increasingly used as an alternative for incandescent lamps, which require high power consumption. A compact self-ballasted fluorescent lamp has the same cap as an incandescent lamp and has a fluorescent lamp as a light emitting unit.
The compact self-ballasted fluorescent lamp includes, for example, an arc tube which is formed by spirally winding a glass tube except for end portions, where the end portions are vertically straight (i.e. leg portions), a holder to which the end portions of the arc tube are attached, a drive circuit unit that is attached to the holder and used for driving the arc tube, and a case that houses the drive circuit unit. In addition, two lead wires extend from each of the end portions of the arc tube which are attached to the holder, and are connected to the drive circuit unit. The lead wires are electrically conductive so as to supply power with a filament.
The drive circuit unit is generally formed in such a manner that a capacitor and a transistor are mounted on a printed circuit board. Furthermore, connection pins are provided on the circuit board, so as to connect the lead wires extending from each end portion of the arc tube to the drive circuit unit. The arc tube and the drive circuit unit are electrically connected to each other by winding the lead wires around the connection pins. Alternatively, the connection pins may be formed by wire-like components which are easily bent. In this case, the arc tube and the drive circuit unit are electrically connected to each other in such a manner that the lead wires and the connection pins are twisted together and then soldered. It should be noted here that all of the connection pins are gathered together in an area, for example, aligned in line, on the circuit board in order to achieve smaller circuit and board sizes.
In an attempt to increase an amount of light emission, a new arc tube has recently been developed for self-ballasted fluorescent lamps. In detail, each end portion of the new arc tube is spirally wound in a spiral direction of the arc tube, unlike the above-described conventional arc tube which has end portions which are vertically straight with respect to the holder. The new arc tube is hereinafter referred to as a spiral arc tube.
When comparing this new spiral arc tube with the conventional arc tube, the new spiral arc tube has approximately four spirals, while the conventional arc tube only has approximately three spirals. Thus, the new spiral arc tube has a larger light emission area. As a result, the new spiral arc tube achieves a larger amount of light emission than the conventional arc tube having the leg portions.
In the spiral arc tube, end portions, from each of which two lead wires extend, are positioned so as to oppose each other when seen from above. As describe above, however, the connection pins are gathered together in an area on the circuit board. Accordingly, lead wires extending from at least one of the end portions of the spiral arc tube need to be bent towards the connection pins, in order to be connected to the connection pins on the circuit board. In other words, one of the end portions of the spiral arc tube is more distant from the connection pins than the other end portion is. This poses the following problem specific to the spiral arc tube in which the end portions are spirally wound in the spiral direction. That is, the lead wires extending from one of the end portions which is positioned more distant from the connection pins need to be bent to be connected to the connection pins.
Despite their weakness, lead wires may be bent many times during a manufacturing process of the spiral arc tube. Accordingly, the lead wires tend to break due to a load of the bending. In addition, since the lead wires need to be long enough to be bent, the lead wires tend to easily contact with each other or with other constituents provided on the circuit board. This means that the new spiral arc tube has a high risk that the lead wires short-circuit. To prevent such a short circuit, the lead wires may be covered with an insulative material. However, this is not desirable since the number of constituents and the number of steps in the manufacturing process increase.
When a main light emitting portion of the spiral arc tube is faced downwards, the two lead wires extending from each end portion of the spiral arc tube are pulled up substantially vertically (in a direction along a spiral axis of the spiral arc tube), to be connected to the circuit board which is positioned directly above the spiral arc tube. Here, more short circuits occur between the lead wires, and between the lead wires and the circuit board in the new spiral arc tube than in the conventional arc tube having the leg portions. The reason for this is explained in the following.
In the conventional arc tube having the leg portions, the two lead wires extend from each vertically straight end portion. When such two lead wires are vertically pulled up towards the circuit board, an interval can be horizontally secured between the two lead wires. Thus, the lead wires do not overlap each other, and a short circuit is therefore less likely to occur. In the new spiral arc tube, on the other hand, each end portion is spirally wound in the spiral direction. Here, the two lead wires extending from each end portion are arranged vertically (in a direction along the height of the spiral arc tube, which is the same direction as the direction along the spiral axis of the spiral arc tube). In this case, when the two lead wires are pulled up substantially vertically to be connected to the circuit board, the lead wires are highly likely to overlap each other. Thus, a short circuit tends to occur between the lead wires. To prevent such a short circuit, the lead wires may be covered with an insulative material. However, this is not desirable since the number of constituents and the number of steps in the manufacturing process increase.
Here, the lead wires are fixed to each end portion of the spiral arc tube by pinching. Specifically speaking, each end portion of the spiral arc tube is melted and then pressed, so as to seal and fix the lead wires. However, this pinching method poses the following problem.
When the lead wires are wound around the connection pins provided on the circuit board to establish electrical connection, the lead wires may be strained. If such is the case, the lead wires may break or sealing portions of the spiral arc tube formed by pinching may break due to a tension produced in the lead wires. These breakages are more likely to happen when the connection pins are formed by wire-like components which are easily bent. This is because, in this case, the connection pins and the lead wires are twisted together using a pair of tweezers or the like, and a higher tension tends to be produced in the lead wires.